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J. Phys. Chem. C 2007, 111, 17700-17704
Preparation of Zinc Oxide Thin Films by Reactive Pulsed Arc Molecular Beam Deposition† Chi-Tung Chiang, Robert L. DeLeon, and James F. Garvey* Department of Chemistry, UniVersity at Buffalo, State UniVersity of New York, Buffalo, New York 14260-3000 ReceiVed: February 1, 2007; In Final Form: April 18, 2007
The structures and properties of ZnO thin films deposited on Si (111), Corning 2947 glass and polyimide substrates by pulsed arc molecular beam deposition (PAMBD) have been studied. PAMBD is an ablation technique that utilizes a high-voltage pulsed electrical arc discharge to provide a plasma source for oxide generation and subsequent material deposition. Metal oxide generation is accomplished by pulsing oxygen gas and using the arc discharge to create a high-temperature plasma. The resulting metal oxide product is expanded and deposited onto a room-temperature substrate situated within a vacuum chamber. In this work, ZnO thin films were grown by the PAMBD method and then characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction, and ellipsometry measurements to determine the film composition, structure, growth rate, and optical properties. SEM studies of ZnO films on different substrates all demonstrated that the PAMBD ZnO films were dense and have columnar structures. XPS revealed a highly stoichiometrical Zn and O (1:1) film composition representing chemically pure films. By observing ZnO thin films deposited on various substrates, we readily see the advantages of low-temperature thin film deposition due to the rapid cooling of the metal oxide product by the molecular beam supersonic expansion. This indicates that the PAMBD process is especially suitable for deposition of these materials upon temperaturesensitive substrates.
Introduction As a transparent semiconductor with high-excitation-binding energy (60 meV) and wide band gap (3.37 eV) at room temperature, ZnO has become a promising material for potential applications in solar cells, liquid-crystal displays, gas sensors, and optoelectronic devices.1-6 Structures of ZnO have also attracted increasing attention because it can be fabricated in a variety of shapes, such as thin films, nanowires, nanorods, and nanoparticles.7-10 There are many pulsed arc-related techniques that have been used to produce ZnO-based thin films, including pulsed filtered vacuum arc deposition, plasma-assisted pulsed arc discharge, pulsed cathodic arc process, and pulsed vacuum arc deposition.11-14 However, high-quality ZnO thin films are usually deposited at a relative high-temperature (200-450 °C) by these techniques making them unsuitable for deposition onto fragile substrates such as plastics or polymers. Electrochemical deposition can be considered to be a low-temperature (60-80 °C) technique for the preparation of ZnO material.15-17 However, such wet chemistry techniques have some limitations, for example, they require electrically conducting substances as substrates, and they may trigger undesirable chemical reactions between the substrate or the various thin film layers and the solutions used for the electrochemical deposition. For temperature-sensitive substrates, one wishes to employ a low-temperature deposition technique that can deposit ZnO films without damaging the supporting material. For example, polymers play an important role in the development of solid-state ionic devices such as batteries, sensors, and flat-panel displays.18,19 Polymers have a number of properties that make them of special interest for electronic materials applications. Polymers are inexpensive, †
Part of the special issue “Richard E. Smalley Memorial Issue”. * To whom correspondence should be addressed. E-mail: garvey@ buffalo.edu.
very flexible materials that can be produced in thin sheets with typical thickness on the order of 100 µm. However, polymer sheets are also temperature-sensitive and have small dielectric constants so that most deposition techniques that use high substrate temperatures or electrodeposition are inappropriate for such polymer substrates. In our previous work, laser-assisted molecular beam deposition (LAMBD), a thin film deposition system, has demonstrated its unique ability to produce high-quality thin films by combining laser ablation and a molecular beam expansion.20-22 This technique utilizes a pulsed 248 nm ultraviolet excimer laser beam for ablating the material of interest to create a hightemperature plasma. A reactive gas is then expended from a molecular beam nozzle to react with the ablated material and also to act as a carrier gas to move the product to be deposited to the substrate. The laser is an expensive and sophisticated component of this system. To replace the laser with a simpler ablation source, we developed the pulsed arc-based deposition (PAMBD) technology.23 This technique uses a high-voltage electrical arc discharge to provide the high-energy source for material ablation and plasma generation with microsecond duration.23 The principle of the PAMBD apparatus is based upon the pulsed arc cluster ion source developed by Siekmann et al., which is a technique that combines a pulsed high-current arc source and a molecular beam expansion.24 They developed this technique as a metal ion source for their mass spectrometric research of metal cluster ions. However, we have found that with the present modifications this technique is also able to produce thin films. Compared to the LAMBD system, the advantages of the PAMBD system is its larger deposition areas and lower costs of equipment and operating expense. Only a small fraction of the cost of the LAMBD system is required to build an entire PAMBD system. However, unlike the LAMBD system, only electrically conduct-
10.1021/jp070898f CCC: $37.00 © 2007 American Chemical Society Published on Web 08/07/2007
Preparation of Zinc Oxide Thin Films by Reactive PAMBD
Figure 1. Schematic drawing of PAMBD source.
ing materials can be used as target rods due to the requirement to strike the electrical discharge. There are many other techniques for thin film deposition that utilize molecular beams such as molecular beam epitaxy (MBE) and ionized cluster beam (ICB) deposition. The MBE technique has the advantage that it is able to generate very thin monolayers with precise composition and operates in the temperature range from 600 to 800 °C; however, the film growth rate is low and requires sophisticated equipment.25 For the ICB deposition technique, the cluster beam is formed by condensation during the expansion of a vapor through an aperture into vacuum, which is very similar to the principles of the LAMBD and PAMBD systems. However, the ICB technique uses a resistor heater or microwave generator to vaporize the material and further ionize clusters with an electron emitter, making the control of the cluster kinetic energy through ionization and acceleration a critical issue in such a deposition process.26 Therefore, development of the PAMBD system is justified by the advantages that it has over other molecular beam thin film deposition techniques. In this current study, ZnO films were deposited on Si (111), Corning 2947 glass and polyimide substrates using the PAMBD technique. While other high-temperature plasma methods can damage temperature-sensitive substrates, the PAMBD process uses a molecular beam expansion to cool the high-temperature ablation products by the conversion of the random thermal motion of the gas particles into a directed mass flow with an increased mean velocity and a decreased translational temperature.27 The PAMBD technique previously has been used to successfully generate metal oxide films for a wide variety of metals.24 The resulting films in this work indicate that the PAMBD process is suitable for deposition of ZnO films upon a variety of substrates, especially temperature-sensitive substrates. Experimental Methods The design and theory of the PAMBD process employed in this work have been described in detail elsewhere; therefore, only a brief description of the PAMBD operating mechanism will be given here. A schematic drawing of the PAMBD source is shown in Figure 1.23,24 The ZnO thin film starting materials are a reactive gas, O2, and a pair of conducting high-purity Zn electrodes. In the PAMBD process prior to triggering the discharge, the gas is pulsed between the ends of a pair of cylindrical electrodes. The gas pulse (∼1ms) lasts 2 orders of magnitude longer than the
J. Phys. Chem. C, Vol. 111, No. 48, 2007 17701 arc (∼10 µs), so a relatively high dynamic pressure is established inside the PAMBD source prior to the discharge and continues during and after the discharge arc plasma. A high-voltage electrical discharge, 1000 V, is struck between the pair of electrodes (anode and cathode) inside a vacuum chamber. For the prototype system, the discharge energy is estimated at 1.5 J/pulse. The distance between the electrodes is kept between 1 and 4 mm for a typical deposition. Relative to the high-vacuum, 10-6 mbar, environment of the vacuum chamber, the gas pulse creates sufficient pressure between the electrodes to support a cold cathode electric discharge. Cold cathode field emission is spontaneous emission of electrons from inside a cold cathode to the space outside of the cathode. The emission has been shown to start at “whiskerlike” surface imperfections with an approximately 0.5 micron radius on the cathode surface. The electric field can be 2 orders of magnitude larger than the average field of the cathode on these imperfections.28 With these enhanced local fields, some electrons will spontaneously tunnel into the vacuum. During field emission, a large electric field is put on the cathode. The high field on the cathode makes the outside potential appear lower. When, from the electron’s frame of reference, the potential outside of the cathode appears lower than the Fermi level of the cathode material, some of the electrons with energies near the Fermi level can tunnel outside of the cathode.29 When a critical current density, about 1012 A/m2, is achieved at a whiskerlike emitter or triple junction, joule heating causes the cathode material to melt, sublimate, and evaporate.28 With a high local pressure of sublimated material and free electrons to ionize the background gas, a cascade of primary emission electrons, secondary electrons, and sublimated electrode materials is accelerated across the electrode potential gap to the anode. In the PAMBD process, this plasma also serves as the highenergy environment that produces a chemical reaction between the ablated material and the gas. At favorable gap widths and source pressures, secondary ionization can form a conducting bridge across the electrode gap, represented by the Paschen curve.28,30 Around the minimum of the curve are values of pressure and electrode gap that give the most abundant secondary electron emission from the source gas and favor plasma arc formation.28 When a conducting bridge is established across the gap, a storage capacitor can discharge across the gap.28 The discharge in the PAMBD source can be categorized as an intense arc. Visible anode spots, which can be observed on the anode during an intense arc, have been observed on the PAMBD anode.31 The spots are caused by large local current densities on the anode. The local current can cause ionization and evaporation of anode material and sputtering of droplets just as in the cathode spots. The ions ejected from the anode are thought to be ionized on the anode surface by energetic electrons from the cathode. Material evaporation is caused by intense local heating on the anode.31 However, the anode of the PAMBD system usually does not show a net mass loss. This is due to some of the cathode material depositing directly upon the anode, which usually results in a net mass gain for the anode. This mass gain is usually on the order of milligrams; therefore, it does not appreciably change the distance between the cathode and anode during deposition. In the PAMBD process, the gas pulse also carries the ablated material from the discharge area onto a substrate to create a thin film. Selecting a reactive gas to pulse through the ablated
17702 J. Phys. Chem. C, Vol. 111, No. 48, 2007
Chiang et al.
cathode material gives a chemically altered thin film containing the starting cathode electrode material, which in this case is ZnO. ZnO thin films starting materials are pure oxygen (reservoir pressure 40 psig, Irish Welding Supply) and a pair of pure zinc rods (Goodfellow, 6.35 mm diameter, 99.9% pure). In this work, three different substrates, Si (111), Corning 2947 glass and polyimide, were placed on the substrate holder several cm in front of the PAMBD source. ZnO thin films were deposited by 1 Hz of pulsed arc, and films were grown for approximately 20 000 pulses that produced films with a 1 cm radius and a thickness of 650, 550, or 700 nm for the Si, glass, or polyimide substrates, respectively. Film thickness and surface morphology were characterized by scanning electron microscopy (SEM). X-ray diffraction (XRD) patterns were obtained for determining the polycrystalline structures. The film composition was analyzed by X-ray photoelectron spectroscopy (XPS), and the refractive index of the film was obtained from a 633 nm Gaertner ellipsometer. The crystalline structures of the ZnO films were measured by a Huber 4-Circle X-ray diffractometer with a Rigaku RU-200 rotating anode generator (15 kW). Results and Discussion Physical properties determined for the ZnO thin films include the film thickness, surface morphology, film structure, crystalline orientation, and refractive index. Figure 2 shows SEM images of ZnO thin films deposited on a silicon wafer, a glass slide, and a polyimide substrate. When a silicon wafer is used as the substrate, the interface between the film and the substrate can be seen clearly to produce a uniform layer of ZnO that adheres to the substrate. No large particles are embedded in the film structure. The average size of small embedded particles is approximately 50 nm in diameter, indicating a fairly good surface morphology. The thickness of this film is approximate 650 nm with a dense and column-like film structure. It appears that the film is built from columns with a diameter ranging from 30 to 50 nm. This observation is in agreement with reported literature studies that show that ZnO material can grow in various types of structures.1-10 The refractive index of the films range from 1.92 to 1.99, which is in good agreement with reported values.32 An SEM image, shown in Figure 2b, for a ZnO PAMBD thin film deposited on a Corning 2947 glass slide indicates a film thickness of approximate 550 nm. A column-like structure is also observed in this film but the spacing is not as obvious as the previous sample, suggesting that the silicon wafer is a better platform to grow such unique film structures. In the SEM image of this film, it is interesting to see a whole section of film was removed from the substrate surface, and the bottom edges can be seen clearly where the film was cleaved to display the cross-section. This indicates that the film’s structure is solid and dense. In the case of ZnO thin films on a polyimide substrate, SEM images show that the ZnO layer is not completely flat and has many cavities. This is consistent with the physical properties of the polyimide substrate because it is a soft and flexible material with a thickness of approximately 100 µm. This indicates that the surface morphology of a given polyimide substrate is easy to damage even by weak forces, and the uneven surface of these films are due to the uneven surface of the substrates themselves. In the SEM image, shown in Figure 2c, the magnification of this image clearly shows the film structure and surface morphology. The thickness of this film is approximately 700 nm with a solid, dense and column-like film
Figure 2. SEM images of zinc oxide thin films deposited on a (a) Si (111) wafer, (b) Corning 2947 glass slide, and (c) polyimide substrate.
structure. The columnar structure is similar to that of ZnO on glass, confirming that the silicon wafer is a more suitable substrate on which to grow such columnar structures. The X-ray diffraction patterns of ZnO films on silicon, glass, and polyimide substrates are shown in Figure 3. In general, the patterns are consistent with a hexagonal wurtzite polycrystalline structure of ZnO. The diffraction peaks detected were (002), (101), and (004). The calculated c parameters from the experimental XRD spectra are a bit larger than the experimental value of bulk-phase ZnO of 5.2066 Å. The values of c are 5.254 Å for glass, 5.254 Å for silicon, and 5.236 Å for polyimide substrates, although the width of the peaks introduces an uncertainty of approximately 0.050 Å. The (002) peak was strongest among these spectra, indicating that the films were predominantly oriented with respect to the c-axis of hexagonal ZnO normal to the substrate surface, but the presence of the (101) peak indicates that the alignment is not complete.1,32 Thus,
Preparation of Zinc Oxide Thin Films by Reactive PAMBD
J. Phys. Chem. C, Vol. 111, No. 48, 2007 17703
Figure 4. Low-resolution 100 Å depth XPS survey scan of film deposited on Si(111) from zinc electrodes with pure oxygen.
TABLE 1: Atomic Percentages of ZnO Films Deposited on Si(111) by the PAMBD System (Data Taken from XPS Analysis) sample (1) C O Zn
AR
20A
48.9 33.7 17.4
14.0 44.0 42.0
sample (2) 100A
AR
20A
100A
50.0 50.0
28.4 42.4 29.2
52.1 47.9
51.0 49.0
measuring the area under each peak present in the XPS survey spectrum. To demonstrate the reliability and consistency of the PAMBD system, two sets of ZnO thin film composition are shown in Table 1. Three XPS spectra were taken for each sample at the “as received” (AR) surface, after sputter down to a 20 Å depth and at a 100 Å depth. Adventitious surface carbon contamination was observed mainly in the AR surface and to a lesser extent at the 20 Å depth. The surface carbon contamination is likely due to material picked up after removal from the deposition chamber and during the transport, storage, and XPS film analysis process. Given that the detection is highly dependent on the scan area and the detection limit is approximately 0.1 atomic %, these two sets of film composition are in good agreement with each other and indicate a 1:1 ZnO stoichiometric film. At 100 Å depth, no carbon contamination was present and the highly stoichiometrical Zn and O (1:1) film composition represents the chemically pure ZnO films. Conclusion
Figure 3. XRD patterns of ZnO films on (a) Si(111), (b) glass slide, and (c) polyimide substrate.
the ZnO films deposited by the PAMBD system can be considered as oriented crystalline structures in agreement with the observation of the column-like structures in the SEM images. An XPS spectrum of a ZnO thin film, see Figure 4, shows that only peaks of zinc and oxygen appear in a low-resolution survey scan. The binding energy for the O(1s) peak was 530.9 eV, which corresponds to the oxygen O2- oxidation state. For the peaks of zinc, the strongest peak is at 1022.1 eV Zn(2p3) and corresponds to the product of ZnOx, which indicates a higher Zn oxidation state. However, according to the atomic percent obtained from the XPS analysis, it was found that the chemical composition of the ZnO films made by the PAMBD system is rigorously stoichiometric. The standard procedure to estimate the atomic percent of film composition is calculated by
Three different substrates have been used to investigate the capability of the PAMBD system. In summary, results of this work support two general conclusions: (1) The PAMBD system is capable of making ZnO thin films. (2) The PAMBD process is suitable for deposition of thin film materials onto temperaturesensitive substrates without damaging the substrate surface because of the rapid cooling of the product by the molecular beam supersonic expansion. SEM studies of ZnO films on different substrates all demonstrated that the ZnO films were dense and have columnar structures. XPS revealed highly stoichiometrical Zn and O (1:1) film composition representing chemically pure films. By observing ZnO thin films deposited on various substrates, we readily see the advantages of lowtemperature thin film depositions due to the rapid cooling of the metal oxide product by the molecular beam supersonic expansion. It was found that the deposition process is readily applied to the production of metal oxide thin films. Applications of this deposition technique are of great interest because one may use a PAMBD system to generate different metal oxide films by
17704 J. Phys. Chem. C, Vol. 111, No. 48, 2007 utilizing a variety of metal electrodes. This great advantage of the PAMBD system allows the deposition of different types of metal oxide films on a substrate layer by layer at relatively low substrate temperatures. Acknowledgment. The authors would like to thank our SUNY at Buffalo collaborators Professor Wayne Anderson, Dr. Li, Professor Wie, and Mr. Dongho Lee of the Electrical Engineering Department and Mr. Peter Bush of the South Campus Instrument Center for sample analysis and helpful discussions. Financial support of this work was provided by MDA Grant DAAD 19-03-C-00003. References and Notes (1) O ¨ zgu¨r, U ¨ .; Alivov, Ya. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; Dogˇan; Avrutin, S. V.; Cho, S.-J.; Morkoc¸ , H. J. Appl. Phys. 2005, 98, 041301. (2) Ashrafi, A. B. M.; Zhang, B.; Ninh, N.; Wakatsuki, K.; Segawa, Y. Jpn. J. Appl. Phys. 2004, 43, 1114. (3) Chen, M.; Pei, Z.; Wang, X.; Sun, C.; Wen, L. J. Mater. Res. 2001, 16 (7), 2118. (4) Look, D. C. Mater. Sci. Eng., B 2001, 80, 383. (5) Suchea, M.; Christoulakis, S.; Moschovis, K.; Katsarakis, N.; Kiriakidis, G. Thin Solid Films 2006, 515, 551. (6) Clarke, D. R. J. Am. Ceram. Soc. 1999, 82, 485. (7) Chou, T.-L.; Ting, J.-M. Thin Solid Films 2006, 494, 291. (8) Liu, C.; Li, H.; Jie, W.; Zhang, X.; Yu, D. Mater. Lett. 2006, 60, 1394. (9) Hu, H.; Yu, K.; Zhu, J. Zhu, Z. Appl. Surf. Sci. 2006, 252, 8410. (10) Amekura, H.; Umeda, N.; Yoshitake, M. Kono, K.; Kishmoto, N.; Buchal, Ch. J. Cryst. Growth 2006, 287, 2. (11) Chen, Y.; Bagnall, D. M.; Zhu, Z.; Sekiuchi, T.; Park, K-t.; Hiraga, K.; Yao, T.; Koyama, S.; Shen, M.Y.; Goto, T. J. Cryst. Growth 1997, 181, 165. (12) Li, B. S.; Liu, Y. C.; Shen, D. Z.; Lu, Y. M.; Zhang, J. Y.; Kong, X. G.; Fan, X. W.; Zhi, Z. Z. J. Vac. Sci. Technol., A 2002, 20, 265. (13) Wang, Y. G.; Lau, S. P.; Lee, H. W.; Yu, S. F.; Tay, B. K.; Zhang, X. H.; Tse, K. Y. Hng, H. H. J. Appl. Phys. 2003, 94, 1597. (14) Villanueva, Y. Y.; Liu, D.-R., Cheng, P. T. Thin Solid Films 2006, 501, 366.
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